Carbon dioxide enrichment device

ABSTRACT

A carbon dioxide enrichment device includes first and second gas diffusion electrodes; an anion exchange membrane; and an electrolytic solution partitioned by the anion exchange membrane. The electrolytic solution contains solvent and solute, and the solute is dissolved to form a dissolved inorganic carbon containing carbonic acid, hydrogen carbonate ions, or carbonic acid ions. The oxygen is consumed by an oxygen reduction reaction on the first gas diffusion electrode, whereby, a dissolved inorganic carbon is formed by a dissolution and ionization reaction of carbon dioxide in the solvent. The dissolved inorganic carbon from the solute or the dissolved inorganic carbon is transported to the second gas diffusion electrode through the anion exchange membrane, and oxygen is formed from the solvent near the second gas diffusion electrode by an oxidation reaction of the solvent on the second gas diffusion electrode, and carbon dioxide is formed from the dissolved inorganic carbon.

TECHNICAL FIELD

The present invention relates to a device capable of enriching carbondioxide by causing dissolution and release of carbon dioxide in anelectrolytic solution utilizing an oxygen-generating/oxygen-reducingelectrochemical reaction.

BACKGROUND ART

Carbon dioxide is a substance widely distributed on earth, accountingfor 0.04% of the atmosphere, which is a compound widely used forindustrial applications. Specific examples of industrial use of carbondioxide include foaming gas for carbonated drinks, bath salts, and fireextinguishing agents; dry ice for cooling; and air for emergencyreplenishment of automobiles. Carbon dioxide in a supercritical state isalso used as an extracting solvent for caffeine, and is further used ina laser that is used in the industrial field, and a carbonic acid gaslaser that is used as a medical laser knife. It is also used as asubstitute for a chlorofluorocarbon refrigerant in a CO₂ refrigerantcompressor.

In the agricultural field, carbon dioxide is used as a carbon dioxidefertilizer for facilitating the growth of plants such as strawberry inforcing culture, and water plant in a water tank for admiration, and isalso used in controlled atmosphere (CA) storage for fresh agriculturalproducts.

As mentioned above, carbon dioxide has been popularly used, and therehas hitherto been a technique of a carbon dioxide facilitated transportmembrane utilizing a difference in a permeability rate of a porouspolymer membrane as mentioned in Non-Patent Document 1, or a techniqueusing a solid molten salt as mentioned in Patent Document 1, as atechnique of enriching carbon dioxide. In the carbon dioxide facilitatedtransport membrane, there exists a need to pressurize a gas to highpressure of about 200 kPa or higher against the carbon dioxidefacilitated transport membrane so as to enrich carbon dioxide, and toreduce the pressure of the side where the enriched gas permeates. Evenin the case of the technique using a solid molten salt, there existed aneed to maintain the device at high temperature of about 600° C. so asto drive the device since the molten salt is used. As mentioned above,there has never existed a device capable of enriching carbon dioxidewith low energy consumption without requiring a large-scale apparatus.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP 11-28331 A

Non-Patent Documents

-   Non-Patent Documents: R. Yegani et. al., J. Membr. Sci., 291, 157    (2007).

DISCLOSURE OF THE INVENTION Problems to be Solved by the invention

The solutions reported in the above prior art documents require a greatdeal of energy, that is, there exists a need to apply heat duringdesorption (release) of carbon dioxide or to maintain high temperatureduring driving, and had a problem that it is impossible to achieve bothenrichment performance of carbon dioxide and low energy consumption.

In light of the above circumstance, the present invention has been madeand an object thereof is to provide a carbon dioxide enrichment devicethat has high enrichment performance, and also enables a significantreduction in energy required during driving.

Means for Solving the Problems

The carbon dioxide enrichment device according to the present inventionis characterized by comprising: a first gas diffusion electrodefunctioning as a cathode; a second gas diffusion electrode separatedfrom the first gas diffusion electrode functioning as an anode; an anionexchange membrane located between the first gas diffusion electrode andthe second gas diffusion electrode; and an electrolytic solutionexisting between the first gas diffusion electrode and the second gasdiffusion electrode to be in contact with the first gas diffusionelectrode and the second gas diffusion electrode and to be partitionedby the anion exchange membrane, being characterized in that theelectrolytic solution contains a solvent and a solute dissolved in thesolvent, and the solute is dissolved in the solvent to form a dissolvedinorganic carbon containing at least one of carbonic acid, hydrogencarbonate ions, and carbonic acid ions; oxygen is consumed by an oxygenreduction reaction on the first gas diffusion electrode, whereby, adissolved inorganic carbon is formed by a dissolution and ionizationreaction of carbon dioxide in the solvent; the dissolved inorganiccarbon derived from the solute or the dissolved inorganic carbon formedon the first gas diffusion electrode is transported to the second gasdiffusion electrode through the anion exchange membrane; and oxygen isformed from the solvent in the vicinity of the second gas diffusionelectrode by an oxidation reaction of the solvent on the second gasdiffusion electrode, and also carbon dioxide is formed from thedissolved inorganic carbon. In other words, when a voltage is appliedbetween the first gas diffusion electrode and the second gas diffusionelectrode, and carbon dioxide and oxygen are introduced into the firstgas diffusion electrode, a reaction occurs as shown in thebelow-mentioned [Chemical Formula 2] on this first gas diffusionelectrode. HCO₃ ⁻ formed by the scheme as shown in the [Chemical Formula2] permeates through the anion exchange membrane and the electrolyticsolution, whereby, a reaction occurs as shown in the below-mentioned[Chemical Formula 3] on the second gas diffusion electrode, and thencarbon dioxide and oxygen are emitted from the second gas diffusionelectrode.

CO₂+H₂O→H⁺+HCO₃ ⁻

O₂+4H⁺+4e ⁻→2H₂O  [Chemical Formula 2]

2H₂O→O₂+4H⁺+4e ⁻

HCO₃ ⁻+H⁺→H₂O+CO₂  [Chemical Formula 3]

As used herein, “dissolved inorganic carbon” means at least one selectedfrom the group consisting of carbonic acid, hydrogen carbonate ions, andcarbonic acid ions, formed by dissolving carbon dioxide in a solvent.

As used herein, “enrichment” means that the concentration of a specificgas is made higher than that in an initial state, and “carbon dioxideenrichment device” means a device capable of making the concentration ofcarbon dioxide higher than that in an initial state with highselectivity.

In the carbon dioxide enrichment device according to the presentinvention, a molar ratio of carbon dioxide and oxygen to be emitted fromthe second gas diffusion electrode is in the range of from 1:0.1 to1:10. Whereby, it enables penetration of carbon dioxide whilemaintaining an influence of driving of the device on the oxygenconcentration at low level.

In the carbon dioxide enrichment device according to the presentinvention, the anion exchange membrane is preferably a permselectivemembrane of monovalent ions. Whereby, it enables selective permeation ofHCO₃ ⁻, leading to achievement of high selective permeation of CO₂.Suppression of permeation of divalent anions facilitates the movement ofmonovalent anions HCO₃ ⁻.

In the carbon dioxide enrichment device according to the presentinvention, an electrolytic solution on a first diffusion electrode(cathode) side partitioned by the anion exchange membrane preferably hasa pH of 7 to 12, and also an electrolytic solution on a second diffusionelectrode (anode) side partitioned by the anion exchange membranepreferably has a pH of 6 to 12, and a difference between the pH of theelectrolytic solution on the first diffusion electrode side and that ofthe electrolytic solution on the second diffusion electrode side ispreferably in the range of from −4 to −0.01. Whereby, a difference inconcentration of OH⁻ or H⁺ arises between the electrolytic solution onthe first diffusion electrode (cathode) side and the electrolyticsolution on the second diffusion electrode (anode) side, and thusfacilitating diffusion of anions HCO₃ ⁻. As a result, it becomespossible to drive the device at lower applied voltage.

In the carbon dioxide enrichment device according to the presentinvention, an electrolyte of the electrolytic solution preferablycontains any one of Li⁺, Na⁺, and K⁺ as cations, and contains HCO₃ ⁻ orCO₃ ²⁻ as anions. Whereby, an electrode reaction corresponding to biasis likely to occur, and thus electrode overvoltage decreases and alsoselective CO₂ permeation occurs.

In the carbon dioxide enrichment device according to the presentinvention, the first gas diffusion electrode and the second gasdiffusion electrode preferably comprise a polytetrafluoroethylene (PTFE)layer, a porous conductor, and an electrode catalyst, thus enabling toimpart water repellency to an electrode surface, and to prevent moisturefrom flowing out of the device. Inclusion of an electrode catalystenables suppression of overvoltage of a reduction reaction of oxygen,leading to a decrease in device driving voltage.

In the carbon dioxide enrichment device according to the presentinvention, the electrode catalyst contains a metal complex or acatalytic component of the metal complex, the metal complex containingany one of a polymer of one or more monomers selected from the groupconsisting of diaminopyridine, triaminopyridine, tetraminopyridine, adiaminopyridine derivative, a triaminopyridine derivative, and atetraminopyridine derivative; or a modified product of the polymer; or acatalytic metal; and the electrode catalyst also satisfying at least oneof the following (i) and (ii):

(i) the content of metal coordinated to a nitrogen atom, analyzed byX-ray photoelectron spectroscopy, is 0.4 mol % or more, and

(ii) the existence of metal coordinated to a nitrogen atom is recognizedby X-ray photoelectron spectroscopy, and also the content of thenitrogen atom is 6.0 mol % or more.

In the carbon dioxide enrichment device according to the presentinvention, the electrode catalyst contains a polymer of one or moremonomers selected from the group consisting of diaminopyridine,triaminopyridine, and tetraminopyridine; or a fired metal complexobtained by firing a polymer metal complex composed of a catalyticmetal; or a catalyst component of the fired metal complex.

In the carbon dioxide enrichment device according to the presentinvention, a specific surface area of the porous conductor is preferably1 m²/g or more, in the BET adsorption measurement. The specific surfacearea is more preferably 30 m²/g, still more preferably 100 m²/g or more,and yet more preferably 500 m²/g or more. Whereby, a reaction area inthe electrode can be increased, thus enabling an increase in currentdensity of an oxidation-reduction reaction of oxygen required to drivethe device.

In the carbon dioxide enrichment device according to the presentinvention, the electrode catalyst may be platinum, nickel-doped carbonnanotube, tungsten oxide doped with copper or nickel, or titanium oxide.

In the carbon dioxide enrichment device according to the presentinvention, the electrolytic solution on at least one of the firstdiffusion electrode side and the second diffusion electrode side is apolymer gel electrolyte. Whereby, leakage of the electrolytic solutionfrom the device can be suppressed, thus enabling a device having highdurability.

In the carbon dioxide enrichment device according to the presentinvention, the electrolytic solution on the first diffusion electrodeside contains a carbonic anhydrase catalyst facilitating a reaction ofthe below-mentioned [Chemical Formula 1]. Whereby, an ionization rate ofChemical Formula 1, thus enabling formation of HCO₃ ⁻ at a higher rate.

CO₂+H₂O→HCO₃ ⁻+H⁺  [Chemical Formula 1]

Effects of the Invention

According to the carbon dioxide enrichment device of the presentinvention, in the constitution including a first gas diffusionelectrode; a second gas diffusion electrode; an anion exchange membranelocated between the first gas diffusion electrode and the second gasdiffusion electrode; and an electrolytic solution existing between thefirst gas diffusion electrode and the second gas diffusion electrode,that is partitioned by the anion exchange membrane; when a voltage isapplied between the first gas diffusion electrode and the second gasdiffusion electrode, and carbon dioxide and oxygen are introduced intothe first gas diffusion electrode, a reaction occurs in this first gasdiffusion electrode, as shown in the below-mentioned [Chemical Formula2], and HCO₃ ⁻ formed by the [Chemical Formula 2] or CO₃ ²⁻ formed byionization, or H₂CO₃ formed by equilibrium permeates through theelectrolytic solution. Whereby, a reaction occurs in the second gasdiffusion electrode, as shown in the below-mentioned [Chemical Formula3], and thus discharging carbon dioxide and oxygen from the second gasdiffusion electrode, leading to the achievement of carbon dioxideenrichment. Therefore, the carbon dioxide enrichment device exert anexcellent effect capable of significantly reducing energy requiredduring driving since it has high carbon dioxide enrichment performanceand there is no need to heat during releasing carbon dioxide.

Accordingly, according to the present invention, it is possible toprovide a carbon dioxide enrichment device that has high enrichmentperformance, and also enables a significant reduction in energy requiredduring driving.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating an embodiment of acarbon dioxide enrichment device according to the present invention.

MODE FOR CARRYING OUT THE INVENTION

An embodiment of a carbon dioxide enrichment device is illustrated inFIG. 1. The carbon dioxide enrichment device includes a first gasdiffusion electrode that functions as a cathode (gas diffusion electrode1); a second gas diffusion electrode (gas diffusion electrode 2)disposed so as to be separated from the first gas diffusion electrode(gas diffusion electrode 1), that functions as an anode; an anionexchange membrane 5; and an electrolytic solution 3.

The anion exchange membrane 5 exists between the gas diffusion electrode1 and the gas diffusion electrode 2 so as to be gas separated from thediffusion electrode 1 and the gas diffusion electrode 2. Theelectrolytic solution 3 swells the anion exchange membrane 5, and existsbetween the gas diffusion electrode 1 and the gas diffusion electrode 2.Namely, the gas diffusion electrode 1 and the gas diffusion electrode 2are in contact with the electrolytic solution 3, and a gas and anelectrolytic solution exist so that a three-phase interface of anelectrode (solid phase), an electrolytic solution (liquid phase or solidphase), and a gas (vapor phase) containing carbon dioxide and oxygen isformed on a surface of the gas diffusion electrode 1 and the gasdiffusion electrode 2, and thus enabling an electrode reaction due tothe gas and the electrolytic solution.

The anion exchange membrane 5 is a membrane that enables selectivelypermeation anions, and particularly enables prevention of the movementof cations contained in a supporting electrolyte, or H⁺ contained in thesolvent. Whereby, unbalanced charge between an electrode 1 and anelectrode 2 caused by an electrode reaction is compensated only by themovement of anions, thus facilitating the movement of HCO₃ ⁻. Therefore,the movement of cations is suppressed by portioning the electrolyticsolution 3 by the anion exchange membrane 5, thus achieving permeationof HCO₃ ⁻ corresponding to an electrode reaction.

Any anion exchange membrane may be used as the anion exchange membrane 5as long as it can exert a function that enables permeation of onlyanions, but does not enable permeation of cations. Examples thereofinclude NEOSEPTA AMX, AHA, and ACM manufactured by Tokuyama Corporation.Preferably, the anion exchange membrane is NEOSEPTA AMX manufactured byTokuyama Corporation.

The gas diffusion electrode 1 and the gas diffusion electrode 2 have astructure including a catalyst layer made of a porous conductor, onesurface of which is subjected to water repellent finishing, and theother surface of which includes an oxygen reduction catalyst supportthereon.

The specific surface area of the porous conductor is preferably 1 m²/gor more, more preferably 30 m²/g or more, still more preferably 100 m²/gor more, and yet more preferably 500 m²/g or more, in the BET adsorptionmeasurement. In case the specific surface area satisfies the abovecondition, a reaction area increases, thus enabling achievement of moreCO₂ permeation amount. In case the specific surface area is less than 1m²/g, sufficient carbon dioxide enrichment performance is not attainedbecause of a small area of the three-phase interface. In order to reducevoltage loss due to surface resistance of the porous conductor, thelower the surface resistance of the porous conductor, the better it is.The surface resistance is preferably 1 kΩ/□ or less, and more preferably200Ω/□ or less. Preferred examples of the porous conductor include acarbon sheet, a carbon cloth, and the like.

Water repellent finishing can be performed by coating a surface of aporous conductor with polytetrafluoroethylene (PTFE). This waterrepellent finishing enables the gas diffusion electrode 1 and gasdiffusion electrode 2 to have a property capable of permeating a gas,but incapable of permeating water, and also the gas diffusion electrodeshave a feature that the gas can diffuse to the catalyst layer.

The catalyst to be supported on the gas diffusion electrode 1 and thegas diffusion electrode 2 is particularly preferably a material thatcatalyzes an oxidation-reduction reaction of oxygen. Examples thereofinclude alloys or complexes containing at least one metal selected fromtransition metals capable of acting as an adsorption site of oxygen,such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag,Ir, Pt, and Au; or compounds containing these metals as a dopant; carbonnanotube; and graphite. Of these catalyst carriers, Pt, Pt/Ru, andcarbon nanotube are preferable because of having comparatively highperformances in light of catalyst performance. Pt/Ru is more preferable.In this case, it is expected that resistance to CO (carbon monoxide)poisoning is improved to obtain a device having long-term stability.

Both the gas diffusion electrode 1 and the gas diffusion electrode 2 aredisposed so that the side, on which a catalyst is supported, is incontact with the electrolytic solution 3, and that the side subjected towater repellent finishing is in contact with an external gas.

The gas diffusion electrode 1 and the gas diffusion electrode 2 areconnected to a DC power 4 through an external circuit. It is necessarythat DC voltage to be applied between the gas diffusion electrode 1 andthe gas diffusion electrode 2 is a voltage that causes a reductionreaction of oxygen in the gas diffusion electrode 1 (cathode), and alsocauses an oxidation reaction of water in the gas diffusion electrode 2(anode). When using water as the solvent of the electrolytic solution 3,the voltage is preferably a voltage that does not cause electrolysis ofwater so as to permanently operate the device, and is preferably avoltage not exceeding 1.2 V that is a voltage to be determined from freeenergy of a decomposition reaction of water, where no electrolysis doesnot theoretically occur. In case there is a loss such as IR drop of anelectrode or an electrolyte, 1.2 V or higher voltage may be applied. Inthis case, the voltage is preferably 10 V or lower. The voltage is morepreferably 5 V or lower, and still more preferably 2 V or lower.

The device is driven by supplying a gas containing carbon dioxide andoxygen to the gas diffusion electrode 1 from the external atmospheresuch as atmosphere. Therefore, the gas diffusion electrode 1 may beprovided so as to increase a contact area with the external atmosphere.

It is preferred that the gas diffusion electrode 1 and the gas diffusionelectrode 2 are disposed oppositely each other. A distance between thegas diffusion electrode 1 and gas diffusion electrode 2, that face eachother, is a distance wide enough to prevent mutual contact betweenelectrodes so as to reduce voltage drop (IR drop) due to solutionresistance as small as possible, and both electrodes are preferablydisposed in proximity as close as possible. In case the electrolyticsolution 3 has sufficiently high ion concentration and also has smallsolution resistance, it is possible to reduce voltage loss due to IRdrop. If there is a fear that both electrodes may be in contact witheach other by close proximity of both electrodes in terms of a structureof the device, a separator may be inserted between the gas diffusionelectrode 1 and the gas diffusion electrode 2. This separator preferablyhas properties that make it to possible to contain an electrolyticsolution 3, and also has insulating properties. In order to preventdiffusion properties of ions existing in the electrolytic solution 3from causing deterioration, the higher the void ratio of the separator,the better it is.

The solvent is preferably a solvent that absorbs carbon dioxide tothereby cause ionization of carbon dioxide. Examples of such solvent arealcohols, an organic solvent, an ionic liquid, and the like, includingwater. Particularly preferred solvent is water, or a mixed solventcontaining water.

The solute to be used in the electrolytic solution 3 is preferably ahydrogen carbonate or a carbonate of an alkali metal, or a hydrogencarbonate or a carbonate of an alkali earth metal. More specifically,the solute is NaHCO₃, KHCO₃, LiHCO₃, Na₂CO₃, K₂CO₃, or Li₂CO₃.

The pH of the electrolytic solution 3 is preferably from 5 to 14. Inorder to adjust the pH of the electrolytic solution 3, an alkalielectrolyte is added to the electrolytic solution 3. Preferred examplesof the electrolyte used to adjust the pH include NaOH, KOH, LiOH, andthe like. In case the pH of the electrolytic solution 3 is lower than 5,the absorption rate of carbon dioxide drastically decreases and thus theabsorption of carbon dioxide is a rate-limiting factor. As a carbondioxide enrichment device is driven, the total inorganic carbonconcentration in the electrolytic solution 3 decreases, resulting indeterioration of performance of the carbon dioxide enrichment device.

Regarding the pH of the electrolytic solution 3, an electrolyticsolution on a first diffusion electrode 1 (cathode) side partitioned bythe anion exchange membrane 5 preferably has a pH of 7 to 12, and alsoan electrolytic solution 3 on a second diffusion electrode 2 (anode)side partitioned by the anion exchange membrane 5 preferably has a pH of6 to 12, and also a difference between the pH of the electrolyticsolution 3 on the first diffusion electrode 1 side and that of theelectrolytic solution 3 on the second diffusion electrode 2 side ispreferably from −4 to −0.01.

In case the pH of the cathode side is lower than 7, the CO₂ absorptionrate drastically decreases and CO₂ is less likely to be absorbed,leading to a decrease in the permeation amount.

In case the pH of the anode side is 6 or lower, the amount of CO₂generated from the electrolytic solution 3 significantly increases andexceeds the adsorption amount on the anode side to thereby release CO₂,resulting in degradation of the electrolytic solution 3. Therefore, inorder to stably drive, it is necessary that the pH of the anode side is6 or higher.

The pH of both anode and cathode sides of the electrolytic solution 3 ispreferably 12 or lower. In case the pH is 12 or higher, a current perunit area decreases due to an increase in viscosity of the solution,thus causing a phenomenon in which the permeation amount decreases.

A difference in the pH of the electrolytic solution 3 separated by anion exchange membrane 5 is preferably from 0.01 to 4. The existence ofthe pH difference facilitates the movement of hydrogen carbonate anionsexisting in the electrolytic solution 3 of the cathode to the anodeside.

It is considered that the principle of the movement of anions due to thepH difference is the same as that of a concentration cell, and thatinside the device goes into a state where about 60 mV is applied per pHdifference of 1.

In case the solute concentration is low, a supporting electrolyte may bedissolved in the solvent so as to improve ionic conductivity of theelectrolytic solution 3. Preferred examples of the electrolyte includeammonium salts such as tetrabutylammonium perchlorate, tetraethylazaniumhexafluorophosphate, imidazolium salt, and pyridinium salt; and alkalimetal salts such as lithium perchlorate and potassium borofluoride. Theelectrolyte also includes salts containing an alkali metal or an alkaliearth metal such as lithium, sodium, potassium or calcium, or an organiccompound having an amino group as a cation, and a halogen ion such aschlorine or bromine, or sulfonium as an anion. In case the electrolyticsolution 3 has sufficient ionic conductivity, there is no need to add asupporting electrolyte.

The electrolytic solution 3 may be gelled and fixed to a predeterminedposition, or may be formed of a gelled electrolytic solution (gelledelectrolytic solution), or a polyelectrolyte. Examples of a gellingagent for gelling the electrolytic solution 3 include a gelling agent, apolymerizable polyfunctional monomer, and an oil gelling agent thatutilize a technique such as a polymer or polymer crossliking reaction.Commonly used substances are applied as a gelled electrolytic solutionand a polyelectrolyte, and preferred examples thereof include avinylidene fluoride-based polymer such as polyvinylidene fluoride, anacrylic acid-based polymer such as polyacrylic acid, anacrylonitrile-based polymer such as polyacrylonitrile, a polyether-basedpolymer such as polyethylene oxide, a compound having an amide structurein the structure, and the like. In case the electrolytic solution 3 isgelled or fixed, the total inorganic carbon concentration of a gelled orfixed electrolytic solution in contact with the gas diffusion electrode1 and a gelled or fixed electrolytic solution in contact with the gasdiffusion electrode 2, that is calculated by [Equation 1], the pH of theelectrolyte, presence or absence and kinds of the supporting electrolyteand the concentration thereof may be different. As the total inorganiccarbon concentration becomes lower, a reverse reaction rate against anionization reaction of an equilibrium reaction shown in [ChemicalFormula 4] decreases. As the pH of the electrolytic solution becomeshigher, an acid dissociation constant pKa value of an equilibriumreaction shown in [Chemical Formula 4] increases. Therefore, in order tofacilitate absorption of CO₂ on the first gas diffusion electrode sideand to facilitate generation of a CO₂ gas on the gas diffusion electrode2 side, it is preferred to decrease the total inorganic carbonconcentration and to increase the pH value in the gelled or fixedelectrolyte in contact with the gas diffusion electrode 1 as comparedwith gelled or fixed electrolyte in contact with the gas diffusionelectrode 2. In this case, a supporting electrolyte is preferably addedto the gelled or fixed electrolyte having smaller ionic conductivity.

(Total inorganic carbon concentration)=[H₂CO₃]+[HCO₃ ⁻]+[CO₃²⁻]  [Equation 1]

CO₂+H₂O

H⁺+HCO₃ ⁻  [Chemical Formula 4]

In a state where a voltage is applied between a gas diffusion electrode1 and a gas diffusion electrode 2 by a DC power 4, if a gas containingcarbon dioxide and oxygen is supplied to the gas diffusion electrode 1,first, a dissolution and ionization reaction of carbon dioxide occurs onthe gas diffusion electrode 1 side, as shown in the following scheme.

CO₂+H₂O→H⁺+HCO₃ ⁻

Using hydrogen ions H⁺ formed by this reaction, an oxygen-reducingelectrochemical reaction occurs, as shown in the following scheme.

O₂+4H⁺+4e ⁻+2H₂O

The higher the concentration of carbon dioxide existing on the gasdiffusion electrode 1 side, the more the reaction amount increases, andthe current value of the carbon dioxide enrichment device increases.

Subsequently, the hydrogen carbonate HCO₃ ⁻ thus formed is partiallyionized to form carbonic acid ions CO₃ ²⁻, and also hydrogen carbonateHCO₃ ⁻ is partially converted into carbonic acid H₂CO₃ by an equilibriumreaction. The thus formed hydrogen carbonate HCO₃ ⁻, carbonic acid ionsCO₃ ²⁻, and carbonic acid H₂CO₃ diffuse to the gas diffusion electrode 2side in the electrolytic solution 3 by concentration diffusion. Sincehydrogen carbonate HCO₃ ⁻, carbonic acid ions CO₃ ²⁻, and carbonic acidH₂CO₃ exist in the electrolytic solution 3, they undergo concentrationdiffusion, together with ions and carbonic acid in the electrolyticsolution 3.

In the vicinity of the gas diffusion electrode 2, hydrogen carbonateHCO₃ ⁻ reaches the gas diffusion electrode 2 by phoresis due toconcentration diffusion and an electrostatic force. On the gas diffusionelectrode 2 side, an oxidation reaction of water as shown in thefollowing scheme occurs to generate oxygen.

2H₂O→O₂+4H⁺+4e ⁻

This reaction causes an increase in the concentration of hydrogen ionsH⁺ in the vicinity of the gas diffusion electrode 2, leading to adecrease in pH. Since this pH change significantly shifts equilibriumamong hydrogen carbonate HCO₃ ⁻, carbonic acid ions CO₃ ²⁻, and carbonicacid H₂CO₃ to the carbonic acid side, hydrogen ions H⁺ react withhydrogen carbonate HCO₃ ⁻ in the electrolytic solution 3 in such manneras shown in the following schemes to form carbon dioxide.

H⁺+HCO₃ ⁻→H₂CO₃

H₂CO₃→H₂O+CO₂

As a result, a mixed gas of oxygen and carbon dioxide is discharged fromthe gas diffusion electrode 2 side. In case the atmosphericconcentration of carbon dioxide (0.04%) is supplied to a gas diffusionelectrode 1, a ratio of oxygen:carbon dioxide is enriched to about 1:1to 2:1.

In case the concentration of carbon dioxide on the gas diffusionelectrode 2 side is high, a reverse reaction of equilibrium as shown inthe following scheme is likely to occur, leading to an increase inovervoltage for causing a reaction on the gas diffusion electrode 2 sideas an anode.

H₂O+CO₂→H⁺+HCO₃ ⁻

Therefore, in order to increase the reaction amount at the same voltage,the lower the concentration of carbon dioxide on the gas diffusionelectrode 2 side, the better it is. The concentration is preferably 5%or less. More preferably, the device is provided with equipment, thatgenerates an atmospheric current to thereby always lower theconcentration of carbon dioxide, on the gas diffusion electrode 2 side.

Describing in detail the above-mentioned dissolution of carbon dioxideas a gas in the electrolytic solution 3, a reaction occurs first by areaction as shown in the following scheme in which carbon dioxidemolecules are surrounded by hydrated water.

CO₂ (g)→CO₂ (aq)

Carbon dioxide dissolved in the electrolytic solution 3 is partiallyconverted into carbonic acid by the addition of water molecules, asshown in the following scheme.

CO₂ (aq)+H₂O (l)→H₂CO₃ (aq)

A rate constant at 25° C. of a forward reaction of this equilibriumreaction is very low, for example, 0.039 s⁻¹, and a rate constant of areverse reaction is 23 s⁻¹. Carbonic acid H₂CO₃ formed by the abovereaction is ionized by an acid dissociation reaction to form hydrogencarbonate HCO₃ ⁻ and hydrogen ions H⁺, as shown in the following scheme.

H₂CO₃ (aq)→HCO₃ ⁻ (aq)+H⁺ (aq)

Hydrogen carbonate HCO₃ ⁻ is further ionized by an acid dissociationreaction to form carbonic acid ions CO₃ ²⁻. Carbonic acid H₂CO₃,hydrogen carbonate HCO₃ ⁻, and carbonic acid ions CO₃ ²⁻ are in anequilibrium state, and an existing ratio of the respective ions in theelectrolytic solution 3 is determined by the pH.

In order to enhance carbon dioxide absorption capacity of the carbondioxide enrichment device, the electrolytic solution 3 preferablycontains a catalyst for a reaction capable of ionizing carbon dioxideand water into hydrogen carbonate HCO₃ ⁻ and hydrogen ions H⁺.Alternatively, it is preferred to support a catalyst for a reactioncapable of ionizing carbon dioxide and water into hydrogen carbonateHCO₃ ⁻ and hydrogen ions H⁺, on a surface on which an oxygen reductioncatalyst is supported, of the gas diffusion electrode 1. Preferredexamples of the catalyst for a reaction capable of ionizing carbondioxide and water into hydrogen carbonate HCO₃ ⁻ and hydrogen ions H⁺include a carbonic anhydrase, a tetra-coordinated complex containing azinc ion Zn²⁺ in the center, and the like.

In the method of enriching carbon dioxide using the carbon dioxideenrichment device, since a mixed gas at a normal temperature supplied toa gas diffusion electrode 1 as a cathode is discharged from a gasdiffusion electrode 2 as an anode at a normal temperature, andabsorption of carbon dioxide into the device is performed in a chemicalmanner, and also movement in the device occurs by phoresis due toconcentration diffusion and an electrostatic force, there is no need tointroduce a great deal of energy. Therefore, it is possible to enrichcarbon dioxide at low cost while suppressing energy consumption.

It is preferred to use, as the electrode catalyst in the presentinvention, a carbon-based catalyst using no platinum. Cost reduction ofthe device can be expected by using no platinum. Herein, the catalystusing no platinum will be described in detail.

1. Electrode Catalyst 1-1. Summary

An electrode catalyst is characterized in that it contains a specificmetal complex as a catalyst component, and also has oxygen reductionreaction (ORR) catalytic activity, durability, and corrosion resistancethat are equal to or higher than those of a conventional electrodecatalyst such as a Pt-based catalyst.

1-2. Constitution

An electrode catalyst contains, as a catalyst component, 1) a metalcomplex having specific physical properties, or 2) a metal complexobtained by subjecting a specific polymer metal complex to a firingtreatment. The constitution of the electrode catalyst of the presentinvention will be specifically described below.

In the present invention, “metal complex” refers to a compound composedof a polymer and/or a modified product thereof, and a catalytic metal,ligands in the polymer or the modified product thereof beingcoordinately bonded with the catalytic metal.

As used herein, “fired metal complex” refers to a compound obtained bysubjecting a polymer metal complex to a firing treatment. As usedherein, “firing (treatment)” refers to a heat treatment at hightemperature.

As used herein, “polymer metal complex” refers to the metal complex in astate of not being subjected to a firing treatment.

“Metal complex” as simply designated herein refers to the fired metalcomplex and the polymer metal complex regardless of whether it hasalready subjected to a firing treatment.

In the electrode catalyst of the present invention, an indispensableconstituent functioning as a catalyst component is, as mentioned below,a metal complex composed of a specific polymer and/or a modified productthereof and a catalytic metal, or a fired metal complex obtained byfiring a metal complex composed of a specific polymer and a catalyticmetal. These metal complexes serving as indispensable constituents inthe electrode catalyst of the present invention are comprehensivelyreferred to as “2-4 aminopyridine polymer metal complex”.

In the present invention, “2-4 aminopyridine polymer” is a generic nameof a compound obtained by polymerzing monomers such as diaminopyridine(C₅H₇N₃), triaminopyridine (C₅H₈N₄) and/or tetraminopyridine (C₅H₉N₅).“Polymer” as simply designated herein refers to “2-4 aminopyridinepolymer” unless otherwise specified. “Modified product thereof” is amodified product of the polymer, which refers to a compound and anoligomer that are obtained by thermal decomposition of a polymer when apolymer metal complex is fired.

The diaminopyridine, triaminopyridine, and tetraminopyridine arecompounds in which hydrogen atoms (H) of pyridine (C₅H₅N) arerespectively substituted with two, three or fours amino groups (—NH₂).The 2-4 aminopyridine polymer may be composed of a monomer alone or acombination of two or more monomers.

Examples of known position isomer of diaminopyridine include2,3-diaminopyridine, 2,4-diaminopyridine, 2,5-diaminopyridine,2,6-diaminopyridine, and 3,4-diaminopyridine; examples of known positionisomer of triaminopyridine include 2,3,4-triaminopyridine,2,3,5-triaminopyridine, 2,3,6-triaminopyridine, 2,4,5-triaminopyridine,and 3,4,5-triaminopyridine; and examples of known position isomer oftetraminopyridine include 2,3,4,5-tetraminopyridine,2,4,5,6-tetraminopyridine, and 2,3,5,6-tetraminopyridine. Each monomercomposing the 2-4 aminopyridine polymer may be any position isomer. The2-4 aminopyridine polymer may be composed only of the same positionisomer, or different two or more position isomers.

In case the 2-4 aminopyridine polymer is composed of two or moremonomers and/or two or more position isomers, the position of eachmonomer and/or position isomer in the 2-4 aminopyridine polymer is notparticularly limited as long as it is polymerizable. For example, it maybe polymerized so that a combination of specific monomers is regularlyrepeated, or may be polymerized at random.

Regarding the polymer metal complex, a ligand included in the 2-4aminopyridine polymer coordinates a catalytic metal. Examples of theatom (ligating atom) that can serve as a ligand in the polymer include anitrogen atom of the pyridine ring and/or a nitrogen atom of an aminogroup. Diaminopyridine, triaminopyridine and tetraminopyridine includethree, four and five nitrogen atoms capable of serving as a ligand in amolecule. Therefore, the 2-4 aminopyridine polymer composed of thesemonomers contains a lot of nitrogen atoms. Accordingly, it cancoordinate a lot of catalytic metals as compared with an electrodecatalyst containing, as a base, a metal complex composed of a polymerthat coordinates a conventional catalytic metal. According to thisfeature, the electrode catalyst of the present invention can have highoxygen reduction reaction (ORR) catalytic activity.

A preferred example of the 2-4 aminopyridine polymer includes“diaminopyridine polymer” in which only diaminopyridine is polymerized.The position isomer composing the diaminopyridine polymer is notparticularly limited and is preferably 2,6-diaminopyridine and/or2,3-diaminopyridine. The reason is that these position isomers cancoordinate the catalytic metal in the polymer in a more stable mannersince nitrogen atoms (N) are most proximally disposed each other. Thediaminopyridine polymer is more preferably a 2,6-diaminopyridine polymerin which only a 2,6-diaminopyridine monomer is polymerized.

The chemical polymerization reaction, that causes bonding of therespective monomers composing the 2-4 aminopyridine polymer, is notparticularly limited, and is preferably anionic polymerization. In casethe polymer is a 2,6-diaminopyridine polymer, it is presumed that thepolymer includes, for example, chemical structure(s) represented by thebelow-mentioned [Chemical Formula 5] and/or [Chemical Formula 6] throughanionic polymerization of 2,6-diaminopyridine.

As used herein, “catalytic metal” is a metal atom or metal ioncoordinated in a metal complex. In the 2-4 aminopyridine polymer metalcomplex as a catalyst component of the present invention, the catalyticmetal is a substance that plays a role in direct catalytic activity. Thecatalytic metal is not particularly limited and is preferably atransition metal. Specific examples thereof include atoms of titanium(Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt(Co), nickel (Ni), copper (Cu), zirconium (Zr), niobium (Nb), molybdenum(Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), hafnium(Hf), tantalum (Ta), tungsten (W), rhenium (Re), osnium (Os), iridium(Ir), platinum (Pt), and gold (Au), or ions thereof. Persons skilled inthe art may appropriately select a proper catalytic metal from thesecatalytic metals according to the intended purposes, taking cost, supplyamount, catalytic activity efficiency, and the like into consideration.In the electrode catalyst of the present invention, the catalytic metalis preferably Cr, Mn, Fe, Co, Ni, and Cu. Of these electrode catalyticmetals, Fe and Co are preferred. The metal complex may be obtained bycoordinating a catalytic metal alone, or coordinating different two ormore catalytic metals. It is considered that Pt and Au are rare andexpensive and are therefore contrary to the object of the presentinvention. However, since use of them in a state of being coordinated ina metal complex enables relative reduction in use amount of Pt ascompared with a known Pt-based catalyst, it is possible to achieve theobject of the present invention. Therefore, they can be included in thecatalytic metal in the present invention.

It is presumed to include, as the structure of the 2-4 aminopyridinepolymer metal complex, a structure represented by the below-mentioned[Chemical Formula 7] in the case of a metal complex in which a2,6-diaminopyridine polymer coordinates cobalt as a catalytic metal(Co-2,6-diaminopyridine polymer, which is often abbreviated herein to“CoDAPP”).

The 2-4 aminopyridine polymer metal complex of the present invention ispreferably obtained by a firing treatment of a polymer metal complex.The reason is that a catalytic metal in the polymer metal complex isstably coordinates to a nitrogen atom by the firing treatment, and thusstable catalytic activity as well as high durability and corrosionresistance can be obtained by chemical hardening.

A mixing ratio of the 2-4 aminopyridine polymer to the catalytic metalsalt in the 2-4 aminopyridine polymer metal complex may be selected sothat a molar ratio of a raw monomer to a catalytic metal atom becomes3:1 to 5:1, and preferably 3.5:1 to 4.5:1.

“Firing temperature” for firing treatment is from 650 to 800° C.,preferably from 680 to 780° C., more preferably from 690 to 760° C., andstill more preferably from 700 to 750° C. The firing treatment can beperformed by a known method for a heat treatment of an electrodecatalyst. For example, a powder of a dry polymer metal complex may befired under a reducing gas atmosphere at the firing temperature for 30minutes to 5 hours, and preferably 1 to 2 hours. It is possible to use,as a reducing gas, for example, ammonia.

As used herein, “specific physical properties” mean physical propertiesexhibited by the 2-4 aminopyridine polymer metal complex in the presentinvention, for example, at least one of the below-mentioned properties(i) and (ii) obtained as a result in which coordaination of a catalyticmetal to a nitrogen atom becomes stable in a polymer metal complex byfiring the polymer metal complex:

(i) the content of metal coordinated to a nitrogen atom analyzed byX-ray photoelectron spectroscopy is 0.4 mol % or more, and(ii) the existence of metal coordinated to a nitrogen atom is recognizedby X-ray photoelectron spectroscopy, and also the content of thenitrogen atom is 6.0 mol % or more.

The content of metal coordinated to a nitrogen atom and the content ofthe nitrogen atom are measured by X-ray photoelectron spectroscopy. Thecontent is the proportion thereof based on the metal complex (based on100 mol % of the metal complex).

The 2-4 aminopyridine polymer may be partially modified by firingthereby to lose the form of a polymer. Such modification is permitted aslong as a fired metal complex can be used as an electrode catalyst, andthus the 2-4 aminopyridine polymer metal complex composing the electrodecatalyst of the present invention can contain a substance in which the2-4 aminopyridine polymer was modified by firing.

There is no particular limitation on the shape of the fired metalcomplex. However, the larger the specific surface area per unit area ofthe electrode catalyst to be supported on a surface of an electrode, thebetter it is. The reason is that it is possible to more enhancecatalytic activity (mass activity) per unit area of the electrode.Therefore, the shape is preferably a particle, and particularlypreferably a powder. The specific surface area of the fired metalcomplex is preferably 100 m²/g or more, more preferably 400 m²/g ormore, and still more preferably 500 m²/g or more. Such specific surfacearea can be measured by a nitrogen BET adsorption method.

The electrode catalyst containing a metal complex can contain a catalystcomponent other than the above-mentioned fired metal complexes. Forexample, known catalysts such as a CoTMPP catalyst may be contained.

1-3. Effects

According to the electrode catalyst, it is possible to coordinate acatalytic metal in a larger amount than that in the case of an electrodecatalyst derived from a metal complex, which contains a conventionalpolymer and a catalytic metal. Whereby, it is possible to have ORRcatalytic activity and durability, that is equal to or higher than, thatof a known electrode catalyst such as a Pt-based catalyst or a CoTMPPcatalyst, and to reduce the use amount thereof.

Since inexpensive metal such as Fe can be used as the catalytic metal inplace of Pt, it is possible to provide an electrode catalyst at lowproduction cost per unit mass, and also to cope with an increase insupply amount of resources caused by mass production of a fuel cell infuture.

2. Method for Producing Electrode Catalyst 2-1. Summary

According to the present production method, an electrode catalyst can beproduced at low cost.

2-2. Method

The method for producing an electrode catalyst includes (a) apolymerization step, (b) a polymer metal complex forming step, and (c) afiring step. The respective steps will be specifically described below.

(a) Polymerization Step

The “polymerization step” is the step of synthesizing a 2-4aminopyridine polymer by anionic polymerization of diaminopyridine,triaminopyridine and/or tetraminopyridine. Monomers to be polymerizedmay be used alone, or two or three kinds of monomers may be used incombination. There is no particular limitation on a mixing molar ratioof the respective monomers when a combination of two or more kinds ofmonomers is polymerized. Persons skilled in the art may appropriatelyselect while taking catalytic activity into consideration. One exampleof preferred monomer use in the polymerization step includesdiaminopyridine alone.

A position isomer of the respective monomers used in the polymerizationis not particularly limited, and is preferably a position isomer inwhich nitrogen atoms serving as a ligand in the monomer molecule areproximally located. For example, when the above-mentioneddiaminopyridine is polymerized as the monomer, 2,3-diaminopyridine or2,6-diaminopyridine in which nitrogen atoms (N) are most proximallylocated are preferable among position isomers of diaminopyridine.

In the present step, the monomer is polymerized by an anionicpolymerization reaction. The anionic polymerization reaction may beperformed using a known method that is conventionally used in therelevant field. For example, the monomer is deprotonated by reactingwith a strong base solution, and then polymerized by using the thusgenerated carbanions as a nucleophilic agent. Examples of the base usedin the strong base solution include sodium hydroxide, potassiumhydroxide, lithium hydroxide, calcium hydroxide, and the like. Thepolymerization temperature and polymerization time are not particularlylimited as long as the reaction proceeds. Usually, the present step canbe achieved by reacting at a temperature of 5 to 40° C. for about 5 to48 hours.

After the polymerization reaction, a solvent is removed bycentrifugation or filtration to recover a polymer. The recovered 2-4aminopyridine polymer is preferably washed with water (includingdeionized water and distilled water), dried and then used in thesubsequent step.

In case the 2-4 aminopyridine polymer, that has already been synthesizedin advance, is used in the above method for producing an electrodecatalyst, production is started from the subsequent polymer metalcomplex forming step without performing the present step.

(b) Polymer Metal Complex Forming Step

The “polymer metal complex forming step” is the step of mixing the 2-4aminopyridine polymer with a catalytic metal salt to thereby coordinatea catalytic metal to the polymer to form a polymer metal complex. Anitrogen atom included in the 2-4 aminopyridine polymer serves as aligand (ligating atom) and is coordinately bonded with the catalyticmetal to form a polymer metal complex.

The “catalytic metal salt” is a salt of the catalytic metal to becoordinated in the metal complex, and specific examples thereof includea hydrochloride, a sulfate, a nitrate, a phosphate, an acetate, and thelike of a catalytic metal. The catalytic metal in the present step maybe metal having catalytic activity in the electrode catalyst and is notparticularly limited, and is preferably a transition metal. Specificexamples thereof include transition metals mentioned in the firstembodiment. Among these salts, salts of Cr, Mn, Fe, Co, Ni, and Cu aresuitable as the catalytic metal salt of the present step, and acatalytic metal salt of Fe or Co is particularly preferable. Specificexamples thereof include iron chloride, iron nitrate, iron sulfide,cobalt nitrate, cobalt chloride, cobalt sulfate, and the like.

A mixing ratio of the 2-4 aminopyridine polymer to the catalytic metalsalt may be selected so that a molar ratio of a raw monomer to acatalytic metal atom becomes 3:1 to 5:1, and preferably 3.5:1 to 4.5:1.Namely, the polymer may be mixed with the catalytic metal salt byselecting the mass of the polymer and the catalytic metal salt so that aratio of (number of moles of a repeating unit composing thepolymer):(number of moles of metal contained in the catalytic metalsalt) falls within the above preferable range. These materials are mixedand dispersed in an appropriate solvent, followed by well stirring, thusmaking it possible to coordinate the catalytic metal or ions thereof inthe 2-4 aminopyridine polymer. It is possible to use, as the medium,water, ethanol, propanol, or a mixed solution obtained by using them incombination, for example, a mixed solution of water and ethanol, orwater and (iso)propanol. The mixing temperature and the mixing time arenot particularly limited as long as the reaction proceeds. Usually, thepresent step can be achieved by reacting at a temperature of 50 to 70°C. for about 30 minutes to 5 hours. In order to well mix the above twosubstances, ultrasonic mixing may also be performed.

When the reaction proceeds, the formed polymer metal complex isprecipitated in the solvent in the form of a solid. After formation ofthe polymer metal complex, the solvent is removed by centrifugation,filtration, or vaporization to recover the polymer metal complex. Inorder to remove the uncoordinated catalytic metal, the polymer metalcomplex may be washed with water (including deionized water, anddistilled water). The polymer metal complex thus recovered may beoptionally powderized using, for example, a quartz mortar.

(c) Firing Step

The “firing step” is the step of firing the polymer metal complexobtained in the polymer metal complex forming step under a reducing gasatmosphere at high temperature to obtain a fired metal complex. Thecatalytic metal moves in the polymer metal complex during the presentstep to thereby prepare a high-durability electrode-active component inwhich a catalytic metal is coordinated in a stable manner.

The firing temperature is from 650 to 800° C., preferably from 680 to780° C., more preferably from 690 to 760° C., and still more preferablyfrom 700 to 750° C. Firing at this temperature enables preparation of afired metal complex as a catalyst component having highoxidation-reduction reaction (ORR) catalytic activity and durability.

In the same manner as in the first embodiment, an ammonia gas can beused as a reducing gas.

The firing treatment can be performed by a known method for a heattreatment of an electrode catalyst. For example, a powder of a polymermetal complex may be fired under a reducing gas atmosphere at the firingtemperature for 30 minutes to 3 hours, and preferably 1 to 2 hours.

After the firing treatment, the fired metal complex is preferablysubjected to a pickling treatment (pre-leaching) using a hydrochloricacid, nitric acid or sulfuric acid solution so as to remove an insolublesubstance and an inert catalyst. After the pickling treatment, the firedmetal complex is well washed with water (including deionized water anddistilled water), recovered by centrifugation or filtration, and thendried, thus making it possible to obtain the objective fired metalcomplex.

The obtained fired metal complex is preferably powderized to form fineparticles using a quartz mortar so as to increase a specific surfacearea.

The fired metal complex obtained in the present step is a catalystcomponent and, therefore, it can also be used as the electrode catalystas it is.

2-3. Effects

According to the above electrode catalyst, it is possible to provide anelectrode catalyst having oxidation-reduction reaction (ORR) catalyticactivity, durability, and corrosion resistance, that are equal to orhigher than those of known electrode catalysts such as a Pt-basedcatalyst and a CoTMPP catalyst, at low cost; and a comparatively simpleproduction method thereof.

3. Conductive Carrier, Support, and Supporting Method 3-1. ConductiveCarrier

The “conductive carrier” refers to a substance that has conductivity,and is also capable of supporting an electrode catalyst. The material isnot particularly limited as long as it is a substance having theabove-mentioned properties. Examples thereof include a carbon-basedsubstance, a conductive polymer, a semiconductor, a metal, and the like.

As used herein, “carbon-based substance” refers to a substancecontaining carbon (C) as a constituent. Examples thereof includegraphite, activated carbon, carbon powder (including, for example,carbon black, VulcanXC-72R, acetylene black, furnace black, and denkablack), carbon fiber (including graphite felt, carbon wool, and carbonwoven fabric), carbon plate, carbon paper, carbon disk, and finestructure substances such as carbon nanotube, carbon nanohorn, andcarbon naocluster.

As used herein, “conductive polymer” is a generic term of a polymercompound having conductivity. Examples thereof include aniline,aminophenol, diaminophenol, pyrrole, thiophene, paraphenylene, fluorene,furan, acetylene, or a single monomer including a derivative thereof asa structural unit, or a polymer of two or more kinds of monomers.Specific examples thereof include polyaniline, polyaminophenol,polydiaminophenol, polypyrrole, polythiophene, polyparaphenylene,polyfluorene, polyfuran, and polyacetylene.

Taking ease of availability, cost, corrosion resistance, durability, andthe like into consideration, suitable conductive carrier is acarbon-based substance, and is not limited thereto in the presentinvention.

The carrier may be composed of a single kind of a carrier, or acombination of two or more kinds of carries. For example, it is possibleto use a carrier using a carbon-based substance in combination of aconductive polymer, or a carrier using a carbon powder as the samecarbon-based substance in combination of a carbon paper.

The shape of the carrier is not particularly limited as long as theshape is capable of supporting the electrode catalyst of the firstembodiment on a surface. For the purpose of enhancing catalytic activity(mass activity) per unit mass in an electrode for fuel cell, the shapeis preferably a powder or fiber shape having a large specific surfacearea per unit mass. The reason is that the carrier having a largerspecific surface area can usually ensure a wider supporting area, andthus making it possible to enhance dispersibility of a catalystcomponent on a carrier surface and to support a larger amount of thecatalyst component on a surface thereof. Accordingly, the shape of afine particle like a carbon powder, and the shape of a fine fiber like acarbon fiber are suitable as the shape of the carrier. A fine powderhaving an average particle size of 1 nm to 1 μm is particularlypreferable. For example, carbon black having an average particle size ofabout 10 nm to 300 μm is suitable as a carrier of the present step.

The carrier also includes a connection terminal with a conducting wirefor connecting a fuel cell electrode with an external circuit.

3-2. Support

The “support” refers to a substance that itself has rigidity and iscallable of imparting a fixed shape to the electrode for fuel cell ofthe present invention. In case the conductive carrier has a powdershape, it is impossible to retain a fixed shape of the electrode forfuel cell by using a conductive carrier including an electrode catalystsupported thereon alone. In case the conductive carrier is in a state ofa thin layer, the carrier itself has no rigidity. In such case, a fixedshape and rigidity of the electrode are imparted by disposing aconductive carrier including an electrode catalyst support thereon on asupport surface.

However, the support is not an indispensable constituent of theelectrode for fuel cell of the present invention. For example, in casethe conductive carrier itself has a fixed shape and rigidity, like acarbon disk, it is possible to retain a fixed shape of the electrode forfuel cell by using a conductive carrier including an electrode catalystsupport thereon. An electrolyte material itself may impart a fixed shapeand rigidity to the electrode for fuel cell in some cases. For example,a thin layer electrode is bonded to both surfaces of a solid polymerelectrolyte membrane in PEFC. In such case, a support is not necessarilyneeded. Accordingly, the support may be optionally added to theelectrode for fuel cell of the present invention.

The material of the support is not particularly limited as long as theelectrode has rigidity enough to retain a fixed shape. The material maybe either an insulator or a conductor. Examples of the material as theinsulation include glass, plastics, synthetic rubbers, ceramics, orpapers or vegetative pieces (including, for example, wood piece), animalfragments (including, for example, ossicle, shell, and sponge) subjectedto a waterproofing treatment or water repellent finishing. The supporthaving a porous structure is more preferable since a specific surfacearea for bonding a conductive carrier including an electrode catalystsupport thereon increases, thus enabling an increase in mass activity ofthe electrode. Examples of the support having a porous structure includeporous ceramics, porous plastics, animal fragments, and the like.Examples of the material as the conductor include carbon-basedsubstances (including, carbon paper, carbon fiber, and carbon bar),metal, conductive polymers, and the like. In case the support is aconductor, it can function as a support and a current collector bydisposing a conductive carrier including an electrode catalyst supportthereon on a surface thereof.

In case the electrode for fuel cell of the present invention includes asupport, the shape of the support usually reflects the shape of theelectrode for fuel cell. The shape of the support is not particularlylimited as long as it can achieve the function of the electrode. Theshape may be appropriately determined according to the shape of a fuelcell. Examples of the shape include approximately plate (including thinlayer), approximately column, approximately sphere, or a combinationthereof.

3-3. Methods (1) Electrode Catalyst Supporting Method

It is possible to use, as the method of supporting an electrode catalyston a conductive carrier, a method known in the relevant field. Examplesthereof include a method of fixing a fired metal complex on a conductivecarrier surface using an appropriate fixing agent. The fixing agentpreferably has conductivity and is not particularly limited. It ispossible to use, as the fixing agent, a conductive polymer solutionprepared by dissolving the conductive polymer in an appropriate solvent,a dispersion of polytetrafluoroethylene (PTFE), and the like. Supportingof the electrode catalyst on a conductive carrier can be achieved byapplying or spraying such fixing agent on a conductive carrier surfaceand/or an electrode catalyst surface to thereby mix them, or dryingafter impregnating in a solution of a fixing agent. It is also possibleto use a method in which a conductive carrier and a fired metal complexare mixed in a solvent such as water, and then a base such as sodiumhydroxide is added to thereby precipitate a fired metal complex on aconductive carrier surface, thus achieving supporting.

(2) Method for Formation of Electrode for Fuel Cell

It is possible to use, as the method for formation of an electrode forfuel cell, a method known in the relevant field. For example, aconductive carrier including an electrode catalyst supported thereon ismixed with a dispersion of PTFE (for example, Nafion (registeredtrademark; DuPont) solution) and the mixture was formed into anappropriate shape, followed by a heat treatment, thus enabling formationof an electrode for fuel cell. In case an electrode is formed on asurface of a solid polymer electrolyte membrane or an electrolyte matrixlayer, like PEFC or PAFC, the mixed solution is formed into a sheet anda solution of a fluororesin-based ion exchange membrane having protonconductivity on a surface of the thus formed electrode sheet, to which amembrane is bonded, is applied or impregnated, and then the sheet islaid on both surfaces of the membrane, followed by hot pressing, thusbonding to the membrane. It is possible to use, as the fluororesin-basedion exchange membrane having proton conductivity, for example, Nafion,Filemion (registered trademark; Asahi Glass Co., Ltd.), and the like.

An electrode for fuel cell can be formed by applying a mixed slurry ofthe mixed solution on a surface of a conductive support such as a carbonpaper, followed by a heat treatment.

The electrode may also be formed by applying a mixed ink or mixed slurryof a solution (for example, Nafion solution) of a proton conductive ionexchange membrane and a conductive carrier including an electrodecatalyst supported thereon on a surface of a support, a solid polymerelectrolyte membrane, or an electrolyte matrix layer.

3-4. Effects

According to the present invention, it is possible to provide anelectrode catalyst having catalytic activity, durability and corrosionresistance, which are equal to or higher than those of a Pt-basedcatalyst, at low cost in a stable manner as compared with a conventionalPt-based catalyst.

The present invention is not limited to the above-mentioned embodimentsand it will, of course, be understood that various modifications can bemade without departing from the scope of the present invention.

EXAMPLES

The present invention will be specifically described by way of Examples.

Example 1 Production of Gas Diffusion Electrode

A commercially available carbon paper (porosity of 70%, thickness of 0.4mm) was used as a conductive porous material. In order to improve gasdiffusivity, a solution containing 30% by weight ofpolytetrafluoroethylene (PTFE) dispersed therein was applied on onesurface of the carbon paper by a bar coater method, and then the resinwas fixed to the carbon paper by firing in a nitrogen atmosphereelectric furnace at a temperature of 340° C. for 20 minutes, and thusallowing to undergo water repellent finishing.

A catalyst paste to be applied on the carbon paper was prepared in thefollowing manner. In a zirconia pot for ball mill, a commerciallyavailable platinum-supported carbon black (supporting 10 wt % Pt/VulcanXC-72) was dispersed in 50 mL of a mixed solvent (2-propanol/water=1/1)so that the content of the carbon black becomes 100 mg. While stirringthe dispersion, a commercially available PTFE was added dropwise andmixed in the form of a Polyflon dispersion (average particle size of 0.3μm). PTFE was added so that a ratio of PTFE to the entire carbon blackbecomes 1:5. The above dispersion containing PTFE added therein wassuction-filtered on the carbon paper, followed by heat sintering throughfiring in a nitrogen atmosphere electric furnace at a temperature of340° C. for 20 minutes to produce a porous gas diffusion electrode 1 anda gas diffusion electrode 2.

(Preparation of Electrolytic Solution)

Sodium hydrogen carbonate NaHCO₃ and sodium hydroxide NaOH weredissolved in ion-exchange water, and the pH value was variously changedunder the condition where NaHCO₃ is saturated.

(Assembling of Device)

A gas diffusion electrode 1 and a gas diffusion electrode 2 weredisposed oppositely each other, and an anion exchange membrane 5(NEOSEPTA (registered trademark) AMX) was interposed into the spacetherebetween, and then the space was filled with an electrolyticsolution 3. The electrolytic solution 3 was sealed so as not to contactwith the open air through the gas diffusion electrode 1 and the gasdiffusion electrode 2, and then these electrodes were connected to a DCpower 4 so that the gas diffusion electrode 1 serves as a cathode andthe gas diffusion electrode 2 serves as an anode. Thereby, the carbondioxide enrichment device was obtained. In order to enable observationof the amount of carbon dioxide discharged from the gas diffusionelectrode 2, a glass container with a tube (having a volume that canachieve 8 mL/cm²) was attached to the gas diffusion electrode 2 side,and then sealed with an O-ring so as not to leak the discharged gas. Acarbon dioxide detector (solid electrolyte sensor type, resolution of0.01%) was attached to a pipe portion of the glass container so as notto leak the discharged gas. Room temperature and the temperature of thesystem were adjusted to 25° C.

The gas diffusion electrode 1 was connected to an anode of a DC power 4and the gas diffusion electrode 2 was connected to a cathode, and thespace between both electrodes was filled with the electrolytic solution3 having a pH of 9.0 adjusted with NaOH, and then DC voltage of 1.2 Vwas applied between both electrodes. Discharge of carbon dioxide fromthe gas diffusion electrode 2 was confirmed by application of a voltage.The amount of carbon dioxide emitted from the gas diffusion electrode 2was confirmed by measuring the concentration of carbon dioxide in theglass container attached to the gas diffusion electrode 2 using a carbondioxide detector. The results are shown in Table 1. The emission amountwas calculated by the following [Equation 2].

$\begin{matrix}{( {{Amount}\mspace{14mu} {of}\mspace{14mu} {emissions}\mspace{14mu} {per}\mspace{14mu} {unit}\mspace{14mu} {area}} ) = {( {{Concentration}\mspace{14mu} {of}\mspace{14mu} {carbon}\mspace{14mu} {dioxide}\mspace{14mu} {in}\mspace{14mu} {glass}\mspace{14mu} {container}} ) \times {( {{Volume}\mspace{14mu} {of}\mspace{14mu} {glass}\mspace{14mu} {container}} )/( {{Area}\mspace{14mu} {of}\mspace{14mu} {gas}\mspace{14mu} {diffusion}\mspace{14mu} {electrode}\; 2\mspace{14mu} {surrounded}\mspace{14mu} {by}\mspace{14mu} {glass}\mspace{14mu} {container}} )}}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

Examples 2 to 4

In the same manner as in Example 1, the pH of the electrolytic solution3 was variously changed by the addition of NaOH and DC voltage of 1.2 Vwas applied between the gas diffusion electrode 1 and gas diffusionelectrode 2. The amount of carbon dioxide emissions was confirmed bymeasuring the concentration of carbon dioxide in the glass containerattached to the gas diffusion electrode 2 using a carbon dioxidedetector. The results are shown in Table 1.

Example 5

In the same manner as in Example 1, except that the electrode catalystin Example 1 was replaced by the below-mentioned electrode catalystusing no platinum in Example 5, examination was made.

Example 1 Preparation of Electrode Catalyst Test Example 1 Preparationof Co-2,6-diaminopyridine Polymer (CoDAPP) Catalyst

A CoDAPP catalyst was prepared by the method according to the secondembodiment of the present invention. A 2,6-diaminopyridine monomer(Aldrich Corporation) was mixed with an oxidizing agent ammoniumperoxydisulfate (APS) (Wako Corporation) in a molar ratio of 1:1.5,followed by mixing. Specifically, 5.45 g of 2,6-diaminopyridine and 1 gof sodium hydroxide were dissolved in 400 mL of distilled water, andthen 27.6 g of APS and 100 mL of water were added. The obtained mixturewas stirred for 5 minutes and 2,6-diaminopyridine was polymerized atroom temperature for 12 hours. After polymerization reaction, theobtained black precipitate was recovered by centrifugation at 3,000 rpm,and then washed three times with distilled water. The precipitate wasdried under vacuum at 60° C. for several hours to obtain a2,6-diaminopyridine polymer.

Subsequently, 5.45 g of a 2,6-diaminopyridine polymer and 3.62 g ofcobalt nitrate (Wako Pure Chemical Industries, Ltd.) were suspended in asolution of 150 mL of water and ethanol (in a mixing ratio of 1:1) sothat a molar ratio of 2,6-diaminopyridine (raw monomer) to cobalt(catalytic metal atom) becomes 4:1. In the same manner, each amount ofthe 2,6-diaminopyridine polymer and cobalt nitrate was calculated fromthe molar ratio so that each molar ratio of 2,6-diaminopyridine tocobalt becomes 6:1, 8:1, and 10:1, followed by mixing. The suspensionwas subjected to ultrasonic mixing for 1 hour using sonicator ultrasonicprobe systems (AS ONE Corporation) and stirred at 60° C. for 2 hours,and then the solution was vaporized. The remaining powder of a polymermetal complex composed of 2,6-diaminopyridine polymer and cobalt wasground in a quartz mortar.

The polymer metal complex was fired under an ammonia gas atmosphere at700° C. for 1.5 hours. The obtained fired metal complex was subjected toan ultrasonic pickling treatment (pre-leaching) using a 12N hydrochloricacid solution for 8 hours, followed by removal of an insoluble substanceand an inert substance and further well washing with deionized water.Finally, the fired metal complex as an electrode catalyst of the presentinvention was recovered by filtration and dried at 60° C.

Comparative Example

Table 1 shows the results obtained by comparing performance of thecarbon dioxide enrichment device shown in FIG. 2 mentioned in Example 1with carbon dioxide enrichment performance of the carbon dioxidefacilitated transport membrane utilizing a difference in a permeationrate of a porous polymer membrane mentioned in Non-Patent Document 1,and carbon dioxide enrichment performance of a device using a solidmolten salt mentioned in Patent Document 1.

TABLE 1 Emission amount Cathode Anode (μL/minute · cm²) side pH side pH[25° C., 1 atom] Example 1 8.3 8.3 12 Example 2 6.2 10 19 Example 3 11.311.3 13 Example 4 6.2 7.6 8 Example 5 8.3 8.3 14 Comparative — — 6Example 1 Comparative — — — (about 0) Example 2

As is shown in the results, carbon dioxide enrich membrane performancewas evaluated at normal temperature under normal pressure which is themost important matter of driving at low energy, and found that thedevice in the present invention has high enrichment performance. Namely,it has been found that both high carbon dioxide enrichment performanceand low energy consumption can be achieved in Examples 1 to 3.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1 First gas diffusion electrode (cathode)    -   2 Second gas diffusion electrode (anode)    -   3 Electrolytic solution    -   4 Power    -   5 Anion exchange membrane

1. A carbon dioxide enrichment device, comprising: a first gas diffusionelectrode; a second gas diffusion electrode separated from the first gasdiffusion electrode; an anion exchange membrane located between thefirst gas diffusion electrode and the second gas diffusion electrode;and an electrolytic solution existing between the first gas diffusionelectrode and the second gas diffusion electrode to be in contact withthe first gas diffusion electrode and the second gas diffusion electrodeand to be partitioned by the anion exchange membrane, wherein theelectrolytic solution contains a solvent and a solute dissolved in thesolvent, and the solute is dissolved in the solvent to form a dissolvedinorganic carbon containing at least one of carbonic acid, hydrogencarbonate ions, and carbonic acid ions; oxygen is consumed by an oxygenreduction reaction on the first gas diffusion electrode, whereby adissolved inorganic carbon is formed by a dissolution and ionizationreaction of carbon dioxide in the solvent; the dissolved inorganiccarbon derived from the solute or the dissolved inorganic carbon formedon the first gas diffusion electrode is transported to the second gasdiffusion electrode through the anion exchange membrane; and oxygen isformed from the solvent in the vicinity of the second gas diffusionelectrode by an oxidation reaction of the solvent on the second gasdiffusion electrode, and carbon dioxide is formed from the dissolvedinorganic carbon.
 2. The carbon dioxide enrichment device according toclaim 1, wherein a molar ratio of carbon dioxide and oxygen to beemitted from the second gas diffusion electrode is in the range of from1:0.1 to 1:10.
 3. The carbon dioxide enrichment device according toclaim 1, wherein the anion exchange membrane is a permselective membraneof monovalent ions.
 4. The carbon dioxide enrichment device according toclaim 1, wherein an electrolytic solution on a first diffusion electrodeside partitioned by the anion exchange membrane has a pH of 7 to 12,wherein an electrolytic solution on a second diffusion electrode sidepartitioned by the anion exchange membrane has a pH of 6 to 12, andwherein a difference between the pH of the electrolytic solution on thefirst diffusion electrode side and that of the electrolytic solution onthe second diffusion electrode side is in the range of from −4 to −0.01.5. The carbon dioxide enrichment device according to claim 1, wherein anelectrolyte of the electrolytic solution contains any one of Li⁺, Na⁺,and K⁺ as cations, and contains HCO₃ ⁻ or CO₃ ²⁻ as anions.
 6. Thecarbon dioxide enrichment device according to claim 1, wherein the firstgas diffusion electrode and the second gas diffusion electrode comprisea polytetrafluoroethylene (PTFE) layer, a porous conductor, and anelectrode catalyst.
 7. The carbon dioxide enrichment device according toclaim 6, wherein the electrode catalyst contains a metal complex or acatalytic component of the metal complex, the metal complex containingany one of a polymer of one or more monomers selected from the groupconsisting of diaminopyridine, triaminopyridine, tetraminopyridine, adiaminopyridine derivative, a triaminopyridine derivative, and atetraminopyridine derivative; or a modified product of the polymer; or acatalytic metal; and the electrode catalyst satisfying at least one ofthe following (i) and (ii): (i) the content of metal coordinated to anitrogen atom, analyzed by X-ray photoelectron spectroscopy, is 0.4 mol% or more, and (ii) the existence of metal coordinated to a nitrogenatom is recognized by X-ray photoelectron spectroscopy, and the contentof the nitrogen atom is 6.0 mol % or more.
 8. The carbon dioxideenrichment device according to claim 6, wherein the electrode catalystcontains a polymer of one or more monomers selected from the groupconsisting of diaminopyridine, triaminopyridine, and tetraminopyridine;or a fired metal complex obtained by firing a polymer metal complexcomposed of a catalytic metal; or a catalyst component of the firedmetal complex.
 9. The carbon dioxide enrichment device according toclaim 1, wherein the electrolytic solution on at least one of the firstdiffusion electrode side and the second diffusion electrode side is apolymer gel electrolyte.
 10. The carbon dioxide enrichment deviceaccording to claim 1, wherein the electrolytic solution on the firstdiffusion electrode side contains a carbonic anhydrase catalystfacilitating a reaction of the below-mentioned [Chemical Formula 1]:CO₂+H₂O→HCO₃ ⁻+H⁺.  [Chemical Formula 1]